Certain polyol fatty acid polyesters have been suggested as low or reduced calorie substitutes for triglyceride fats and oils used in foods. For example, nonabsorbable, nondigestible sugar fatty acid esters or sugar alcohol fatty acid esters having at least 4 fatty acid esters groups with each fatty acid having from 8 to 22 carbon atoms have been used as partial or full fat replacers in low calorie food compositions. See Mattson and Volpenhein; U.S. Pat. No. 3,600,186; Issued Aug. 17, 1971. Likewise, certain intermediate melting polyol polyesters have been developed that provide passive oil loss control, while at the same time reducing waxiness in the mouth. See Bernhardt; European Patent Application Nos. 236,288 and 233,856; Published Sep. 9, and Aug. 26, 1987, respectively. Blends of completely liquid polyol polyesters with completely solid polyol polyester hardstocks, preferably esterified with C.sub.10 to C.sub.22 saturated fatty acids (e.g. sucrose octastearate) have also been proposed. Other blends of liquid polyol polyesters and solid nondigestible fats are also known. See for example, Elsen et al, U.S. Pat. No. 5,422,131, 1995, Jandacek; U.S. Pat. No. 4,005,195; and Jandacek/Mattson; U.S. Pat. No. 4,005,196; Both issued Jan. 25, 1977.
A number of different processes have been disclosed in the art for preparing these highly esterified polyol fatty acid polyesters, in particular sucrose polyesters. One such process involves a solvent-free, essentially two-step transesterification of the polyol (e.g., sucrose) with the fatty acid esters of an easily removable alcohol (e.g., fatty acid methyl esters). In the first step, a mixture of sucrose, methyl esters, alkali metal fatty acid soap and a basic esterification catalyst are heated to form a melt. The amount of methyl esters used is such that the melt forms primarily partial fatty acid esters of sucrose, e.g., sucrose mono-, di- and/or triesters. In the second step, an excess of methyl esters are added to this melt which is then heated to convert the partial sucrose esters to more highly esterified sucrose polyesters, e.g., sucrose hexa-, hepta-, and particularly octaesters. See, for example, U.S. Pat. No. 3,963,699 (Rizzi et al.), issued Jun. 15, 1976; U.S. Pat. No. 4,517,360 (Volpenhein), issued May 14, 1985; and U.S. Pat. No. 4,518,772 (Volpenhein), issued May 21, 1985, which disclose solvent-free, two-step transesterification processes for preparing highly esterified polyol fatty acid polyesters, in particular highly esterified sucrose polyesters.
In some processes for preparing highly esterified polyol fatty acid polyesters, all of the fatty acid methyl esters are added to the polyol (e.g., sucrose) at the beginning of the reaction, i.e. a one-step addition process. See, for example, U.S. Pat. No. 4,611,055 (Yamamoto et al.), issued Sep. 9, 1986. Like the two-step processes, partial fatty acid esters of sucrose are formed first and are then converted to more highly esterified sucrose polyesters. Accordingly, these single-step and two-step processes are collectively referred to hereinafter as "two-stage" transesterifications, wherein the "first stage" involves the formation of the partial esters and wherein the "second stage" involves the conversion of the partial esters to more highly esterified polyesters.
Alternatively, highly esterified polyol polyesters may be prepared by two stage solvent-based processes, (see, for example, U.S. Pat. No. 4,954,621 issued to Masaoka et al.), or one stage solvent-based or solvent free processes, see for example, U.S. Pat. No. 4,968,791, (Van Der Plank), issued Nov. 6, 1990; U.S. Pat. No. 5,079,355 (Meszaros Grechke et al.) issued Jan. 7, 1992; or U.S. Pat. No. 5,071,975 (Ver der Plank et al.) issued Dec. 10, 1991.
The methyl esters used to prepare polyol polyesters can be prepared by the transesterification of triglyceride oils and fats with methanol in the presence of an alkaline catalyst. After the transesterification reaction, a crude glycerine layer comprising glycerine formed in the transesterification reaction, soap formed by the catalyst, catalyst, some methyl esters and methanol, is separated from the fatty-acid methyl ester layer. The methyl ester layer is then purified by any suitable recovery method, such as, e.g., distillation. Processes of this type have been described in U.S. Pat. Nos. 2,383,596, 2,383,579, 2,383,580, 2,383,596, 2,383,599, 2,383,601, 2,383,602, 2,383,614, 2,383,632 and 2,383,633 and in the European Patent 0 164 643. An extra esterification step before recovery, but after separation of the fatty acid methyl ester layer from the glycerol layer may optionally be used to produce high yields of high purity fatty acid methyl esters. See European Patent 391 485.
Methyl esters are a cheaper carboxylic acid source than acid chlorides or anhydrides, and they are sufficiently reactive to provide a good source of fatty acids for complex esterification reactions. The economics of the reactions are such that the relatively inexpensive cost of methyl esters outweighs any added processing costs. The lower alkyl alcohol group is chosen because the alcohol can be easily removed in the subsequent transesterification reaction through vacuum distillation or reducing the partial pressure of the alcohol using a nitrogen or inert gas sparge, forcing the transesterification reaction to completion.
Typically, methyl esters of fatty acids are prepared from the naturally occurring fatty acids sources, usually triglycerides from vegetable or animal sources. The methyl alcohol replaces the glycerine. The resultant mixture of methyl esters are easily fractionated, providing a purified source of fatty acids.
In a standard polyol polyester synthesis of this type, the polyol is reacted with a lower alkyl fatty acid ester in the presence of a catalyst and under an inert atmosphere. The inert gas stream from this transesterification reaction contains the released lower alkyl alcohol which is removed continuously from the reaction to drive the polyol polyester synthesis to completion. The transesterification reaction can be coupled with another transesterification process to make a lower alkyl, e.g., methyl, esters of fatty acids through a reaction using gaseous alcohols derived from the polyol polyester synthesis reaction as a source of the lower alkyl alcohols.
It has been found that the alcohol, diluted with nitrogen or other inert gas carrier can be reacted with a fatty acid ester, preferably a triglyceride, to form the corresponding methyl or lower alkyl fatty acid ester in a very efficient process. Preferably the reaction is run in a reactive adsorption column, but it can be done in a batch process. The recovered inert gas sparge from the polyol polyester synthesis is used to make the starting methyl esters for the synthesis. At the same time, over 90%, and up to 99.7%, of the methanol is removed from the inert gas stream. The alcohol free or reduced alcohol nitrogen (inert gas).can then be continuously recycled to the polyol polyester synthesis. This ability to recycle nitrogen significantly improves the economics of these reactions.
A key economic driver for this process is the integration or close coupling of methyl ester synthesis and transesterification reactions which use these esters as a fatty acid or carboxylic acid source. Traditionally methanol can be recovered from the inert gas stream by condensation, absorption into organic solvents, (e.g. triethylene glycol) or adsorption onto activated carbon. This reaction when coupled with a polyol polyester synthesis eliminates a separate methanol recovery system, eliminates handling of methanol and partially reduces the discharge of methanol into the environment.
It is an object of this invention to provide an improved method for making polyol polyesters. It is a further object of this invention to provide a method for making methyl esters of fatty acids through a transesterification reaction using gaseous methanol in a reactive adsorption column which is coupled with a polyol polyester synthesis.